Method of increasing the resistance of metals to corrosion



METHOD OF INCREASING THE RESISTANCE OF METALS TO CQRROSION Erik Birger Bengtsson and Olof Ingvar Olsson, Karlskoga,

Sweden, assignors to Aktiebolaget Bofors, Bofors, Sweden, :1 Swedish corporation No Drawing. Application October 28, 1952 Serial No. 317,362 7 1 Claims priority, application Sweden March 26, 1952' 4 Claims. 01. 148--6.15)

In such chemical industry where corroding agents are used, the choice'of construction material for reaction vessels will often cause great diificulties. The different brands of acidproof steel still do not satisfy all the requirements on resistance to acids, such as nitric acid and sulphuric acid. In particular in such reaction vessels made of acidproof steel, where the reaction occurs at high temperature, the corrosion is considerable in the presence of the above mentioned acids;

This invention relates to a method according to which the resistance of metals to corrosion is increased. By treating the metal with nitric acid, nitrates and/ or nitrous gases in the presence of inorganic metal phosphates and/or organic phosphates at elevated temperature and/or under pressure, an increased resistance to the above mentioned corrosive attack is obtained.

In the following some examples are given of the method according to the inventionand some figures of results achieved. What henceforward below will be said is however not intended to. limit the extent of this invention.

Example 1 The charge was allowed to cool and after three hours the autoclave was discharged. p-Nitro benzoic acid was formed during the reaction. Y

The acidproof steel sample had lost some weightbut",

. it has been found-that metals treated in this way are resistant to sulphuric acid, according to Example 2.

Example 2 Acidproof metal plates, treated according to Example 1, were immersed 'into 10% sulphuric acid, which was refluxed for hrs. After-this time a loss of weight of only 0.1 g./m. /h. could be detected. Even to 50% sulphuric acid the resistance was. good. Untreated plates of the same material were submitted to the same acid solutions. These plates were rapidly attacked by the acid the evolutionof hydrogen.

Patented Mar. 8 1960 'ice Example 3 Acidproof metal plates treated according to Example 1, were immersed into acetic acld and boiledin this acid for 24 hrs. The loss in Weight was after this treatment 0.01 g./m'.' /h. Under the same conditions untreated plates lost 4 g./m. /h.

Example 4 C. and at a pressure of about 40 kg./crn. for one hour.

By. the oxidation nicotinic acid is formed amongst other products. Plates treated in this way showed the same resistance towards nitric, sulphuric and acetic acid as those described in Examples l and 2.

, Example In the oxidiation of a mixture of pyridine bases with nitric acid at a pressure of 30 kg./crn.. and about 200 i C., a 33% nitric acid used containing 0.5% ortho phosphuric acid. Acidproof steel samples sufiered hereby no. loss in weight, but gained slowly in weight.

The test plates in the above mentioned experiments 7 were primarily of acidproof steel of different composition. In all the cases the final result has been in agreement with what has been stated in the above related examples.

We claim:

1. Method of enhancing the corrosion resistance of acidproof steel'which comprises subjecting said steel to the action of dilute nitric acid, the concentration of the acid being about 33%, in the presence of amember of the group consisting of asalt of phosphoric acid and phosphoric acid at a temperature above C. and at a pressure within the approximate range of from 3 to 40 kg. per square cm.

2. Method of enhancing the corrosion resistance of acidproof steel which comprises subjecting said steel to the action of a dilute solution of nitric acid in the presence of 'a member of the group consistingof a salt of phosphoric acid and phosphoric acid at a temperature above 185 C. and under super-atmospheric pressure.

3. Method of enhancing the corrosion resistance of' acidproof steel which comprises subjecting said steel to the action of dilute nitric acid in the presence of a small amount-of phosphoric acid at temperatures above 185 7 C. and under super-atmospheric pressure.

4. Method of enhancing the corrosion resistance of acidproof steel which comprises subjecting said steel to the action of dilute nitric acid in the presence of an alkali metal phosphate at temperatures above 185 C.

and undersuper-atmospheric pressure.

References Cited in the file of this patent UNITED STATES PATENTS \Attesting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,927,873 March 8 1960 Erik Birger Bengtsson et al.

It is herebjr certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 37, for "within the approximate range of from 3" read within the approximate range of from 30 Signed and sealed this 20th day of September 1960.

( SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Commissioner of Patents 

1. METHOD OF ENHANCING THE CORROSION RESISTANCE OF ACIDPROOF STEEL WHICH COMPRISES SUBJECTING SAID STEEL TO THE ACTION OF DILUTE NITRIC ACID, THE CONCENTRATION OF THE ACID BEING ABOUT 33%, IN THE PRESENCE OF A MEMBER OF THE GROUP CONSISTING OF A SALT OF PHOSPHORIC ACID AND PHOSPHORIC ACID AT A TEMPERATURE ABOVE 185*C. AND AT A PRESSURE WITHIN THE APPROXIMATE RANGE OF FROM 3* TO 40 KG. PER SQUARE CM. 